Proton/Helium spectral anomaly and other signatures of diffusive shock acceleration/propagation in/from SNR

Transcription

1 Proton/Helium spectral anomaly and other signatures of diffusive shock acceleration/propagation in/from SNR M. Malkov CASS/UCSD Collaborators: P. Diamond, R. Sagdeev 1

2 Supernova Remnant Shocks- Cosmic Ray Connection S. Reynolds 08 From: Warren et al

3 Linking Cosmic Rays with their putative accelerators in SNR the proof can only be indirect and not easy orbit scrambling in stochastic magnetic field of the galaxy -arrival direction is random, no association with accelerator physics of the spectrum transformation during propagation is not completely understood, but tools available, e.g., GALPROP direct observation of the SNRs detect only the secondary emission of CR: synchrotron, Bremsstrahlung, Inverse Compton (electrons); p-p collisions pi-0 decay gamma rays: easy to confuse with IC from electrons, secondary e s, Bremsstrahlung 3

4 Linking Cosmic Rays with their putative accelerators in SNR Conventional proof strategy: fit broad-band (from radio to gamma) emission to a diffusive shock acceleration (DSA) model: usually a power-law with exponential cut-off similar for all species (modulo e-losses) empirical e/p ratio (from CR background) Alternative/complementary strategy: look for the signatures of the underlying CR acceleration mechanism (DSA) or propagation effects: Spectral slope changes at characteristic CR energy spectral break Sharp angular anisotropy (Milagro/IceCube) Paolo Desiati, Tue talk Elemental bias (slightly different slopes) p/he anomaly, Pamela 4

5 Why study spectral breaks? Long standing community paradigm for SNR emission: power-law spectrum of parent particles (i.e. DSA generated, roughly E^-2) exponential cut-off (particle losses, limited acceleration time), e.g., Drury, Aharonian and Voelk, 94 while plausible overall, fails in some cases: e.g., search campaign for the TeV emission from a number of SNRs - negative (Buckley et al 1998) 5

6 When acceleration mechanism refinement is needed? precisely when hadronic scenario is expected: dense environments (i.e. molecular clouds, swept-up shells, high target density for p-p collisions) DSA works efficiently only when particles are scattered by self-generated waves, but in dense, partially ionized gases, wave propagation can be very different from that in the fully ionized medium -caveat: as soon as a break in gamma emission forms, the high-energy portion of the spectrum may be buried under the IACT sensitivity thresholds - result: a number of promising SNRs (including W44 and IC443) have been tested negative for the TeV gamma ray emission in early days (Buckley et al 98) environmental DSA modification suggested (MM, P. Diamond and R. Sagdeev, 2005) impairment of CR confinement in a dense gas (MC) due to non-propagation of Alfven waves lack of resonant wave-particle interaction --> break (curvature) in the particle and radiation spectra in the TeV energy range (for SNR RX J ) exact position of the break depends on a unique combination of plasma parameters in the emission zone (GeV-TeV, specified below) NB: non-environmental, intrinsic breaks also studied MM, P.Diamond, 06 6

7 Observations of spectral breaks Spectral break around a few GeV detected by Fermi-LAT (Abdo et al : W44, IC443, W28, W51C) SB found consistent (along with the cut-off) with RXJ 1713 HESS data (Aharonian et al 2006); SB suggested MM, Diamond and Sagdeev (2005) If SB was included in gamma-emission estimates (Drury et al94; Buckley 98), non-detection of TeV photons from some of the remnants (e.g., W44, IC443, W51) by IACTs would be no surprise 7

8 Physical phenomena associated with SB are not included in most models Fermi-LAT + HESS, RXJ 1713, Abdo et al, 2011 (see also F. Aharonian, ICRC 2011) IC/pp- emission origin oft- favored/disfavored according to the fit quality (criteria?) pp-scenario is viable precisely when the environment is complicated (strongly inhomogeneous, irregular, weakly ionized medium) folding a simple (test-particle or NL) DSA spectrum through the gas with properly distributed parameters, generates almost arbitrarily shaped emission spectrum inverse problem may be plausible: given emission spectrum, find the gas parameter distribution sensible approach: look for a robust spectral feature, such as break with specific, predictable break energy and slope variation in a narrow energy-band 8

9 Suppression of low-energy (i.e. FERMI gamma-band, X-ray) emission: p p max Κ(p)/V shock Accelerated particles only high energy particles light up distant clumps by layering the clumps upstream, the spectrum may be shaped at will close-to-shock clumps may Downstream Dense gas clumps 9

10 Theoretical basis for obtaining parent particle spectrum: convection-diffusion equation Two-way balance Three-way balance q is well constrained: 3.5 < q < 4 (< q_sub, close to the sub-shock) 10

11 Key element of spectrum steepening (break): waveparticle interaction in the foreshock plasma waves in strongly ionized (closer to the shock) medium propagate freely in a broad frequency range at the Alfven speed When ion-neutral collision frequency is higher (deeper into the cloud), neutrals are entrained by the waves which are still able to propagate with a lower speed between these two regimes Alfven waves are heavily damped and even disappear altogether for sufficiently small Kulsrud & Pearce 69; Zweibel & Shull 82 evanescence wave number range is then bounded by 11

12 cyclotron resonance condition spectrum steepening cont d frequency range where the waves cannot propagate translates into the parallel momentum range spectral break must form at the photon energy corresponding to the particle momentum 12

13 spectrum break cont d locally isotropic component of new proton distribution (effective phase space dimension of emitting particles is reduced by one): slope of the particle momentum distribution becomes steeper by exactly one power above any power-law distribution upon entering an MC, turns into 13

14 Position of the break in the particle spectrum -can be calculated using and i-n collision frequency (e.g., Draine and McKee 93, Drury et al 96) break momentum: depends on the magnetic field strength ion density frequency of ion-neutral collisions ionization rate SNR W44: break in the photon spectrum is observed at about 2 GeV, which places the break on the proton distribution at about 7GeV/c constraints the above combination of parameters 14

15 Case Study W44: Acceleration model and proton spectrum need to determine the degree of nonlinear (NL) modification of the shock structure can be calculated self-consistently, given the shock parameters and the particle maximum momentum P_max in the case of a broken spectrum, P_br has the role of P_max in cases of low maximum momentum, the shock modification is weak, so the spectrum is more likely to be in a slightly NL, almost test particle (TP) regime hint from radio observations (Castelletti et al, 07) : q=1.75 (however, Uchiyama et al 10 fit flat radio without NL acc n) calculate both NL and TP regimes 15

16 W44: Acceleration model and proton spectrum cont d 16

17 Gamma-ray emission spectrum collisions of accelerated protons with the protons of the ambient gas resulting in the following spectrum of π0-mesons: decay of π0 results in the gamma emission spectrum use Karlsson and Kamae, 2008 handy p gamma conversion tool (many thanks!) 17

18 Gamma-ray emission spectrum SNR W44 -Circles: Fermi-LAT, Abdo et al, Squares: Agile, Giuliani et al solid line: TP with break q=2 3, no cut-off to be specified to fit the data -dashed line: NL DSA spectrum with break, requires cut-off at 300 GeV to fit; MM, P. Diamond, R. Sagdeev, NB: only one decade in energy, higher fit quality - Δq < 1 would require tweaking MC filling factor (e.g. W51C, Carmona ICRC11, MAGIC) 18

19 Gamma-ray emission spectrum SNR W44 Uchiyama, et al 2010: reacceleration of preexisting CR + secondaries is sufficient to power gamma emission through p-p collisions break at ~7GeV with Δq=1 is essential to fit the Fermi data 19

20 Spectral Break: Conclusions recent observations of supernova remnants W44, W28, IC443 and W51C by Fermi-LAT (RX J1713, HESS?) indicate breaks in photon spectra and support the idea that the bulk of galactic cosmic rays is accelerated in SNRs by diffusive shock acceleration when a SNR expands into weakly ionized dense gas, significant revision of the mechanism is required strong ion-neutral collisions in the remnant surrounding lead to the steepening of the energy spectrum of accelerated particles by exactly one power (less if MC filling factor is low) gamma-ray spectrum generated in collisions of the accelerated protons with the ambient gas is calculated and successfully fitted to the Fermi-LAT/Agile(W44) data 20

21 Pamela p/he anomaly The PAMELA orbital telescope revealed deviation between helium and proton spectra deemed inconsistent with DSA, Adriani et al 2011 points to initial phase of acceleration where elemental similarity does not apply 21

22 Elemental invariance Rigidity Equations where Diffusive shock acceleration (DSA) mechanism predicts a power-law distribution for accelerated particles where 22

23 Pamela puzzle almost identical (three digits in the indices) convex shapes at 5< R < GV with a likely roll-over towards the right end of this interval sharp dip at R = GV upturn with nearly the same slope at R > GV. 23

24 Suggested explanations Zatsepin & Sokolskaya (2006): three different types of accelerators contribute to the observed spectra Ohira & Ioka (2011): outward-decreasing helium abundance in certain SNR explosion sites, such as super-bubbles, may also result in harder helium spectra, as generated in stronger shocks Drury (2011): He is neutral in the ISM when it is processed by old and weak SNR shocks, and it is ionized when the SNRs are young and the shocks are strong Blasi & Amato 2011: spallation processes lead to deficiency of He at lower energies (longer confinement time in the Galaxy) 24

25 GALPROP analysis (Vladimirov et al. 2011): does not predict sufficient He spallation effect (code, run for standard set of parameters) by stretching inelastic cross sections and gas density the effect can be achieved, but at the expense of overproduction of CR secondaries conclusion: need to assume that nuclei heavier than H are injected into the ISM with a harder spectrum than protons 25

26 Occam s approach to resolve Pamela puzzle within DSA single shock propagating into fully ionized homogeneous ISM suggested mechanisms cannot explain 0.1 difference in the slopes p/he spectral anomaly seriously undermines or even dismiss the DSA as a viable source of galactic CRs 26

27 Main idea: preferential injection of He into the DSA for higher Mach numbers Mach number Collisionless plasma SNR shocks (Sagdeev, 1979; Kennel et al., 1985; Papadopoulos, 1985; MM, 1998) inject more He2 when they are stronger and so produce harder He spectra injection bias is due to Alfven waves driven by protons, so He2+ ions are harder to trap by these waves because of the larger gyroradii. 27

28 Physics of He preferential injection Pitch-angle/gyro-phase plane downstream upon crossing the shock, both protons and He randomize their downstream frame velocity (not energy) proton gyro-radius is a half of that of He2+ (for given velocity V_shock) He ions have better chances to return upstream since protons are retained by the downstream waves more efficiently. injection rates of both species decrease with the Alfven Mach but the proton injection decreases faster 28

29 How does the He-biased injection translates into observed spectra? Injection scalings: with with Contribution to the total number of CRs deposited inside the forward shock over the Sedov-Taylor SNR evolution phase: 29

30 Result take the ratio p/he, approximately: (from MM, P. Diamond, R. Sagdeev 2011, astro-ph) heliospheric modulation not included higher rigidity end needs some adjustment of injection rates 30

31 Summary and Outlook the p/he ratio at ultrarelativistic rigidities, as opposed to the individual spectra, is not affected by the CR propagation, if collisions are negligible telltale signs, intrinsic to the particle acceleration mechanism reproducible theoretically with no free parameters collisionless shock theory needs to be advanced to predict the p/he ratio at higher rigidities (ATIC, CREAM, AMS-02) PIC simulations will be very instrumental in computing p and He injection scalings with Mach number (recent Spitkovsky et al work) 31